A semiconductor IC (Integrated Circuit) device includes a fine trench structure. A typical example of the fine trench structure is a STI (Shallow Trench Isolation) structure. The STI structure is a device isolation region that isolates active regions of a semiconductor device from each other. The STI structure is formed by forming a fine trench in a silicon substrate and filling the fine trench with a dielectric material.
A SOD (Spin-On Dielectric) material is known as an example of the dielectric material that fills the fine trench. In particular, attention is focused on an inorganic polymer containing PHPS (perhydropolysilazane: SiH2NH) as the principal ingredient. For example, the PHPS may be modified into silicon oxide (SiO2) when being sintered in a water-vapor atmosphere. A reaction formula thereof is as follows:SiH2NH+2H2O→SiO2+NH3+2H2 
However, as the high integration of the semiconductor device is improved, a width of the trench which is formed in the device isolation region becomes smaller, which results in a high aspect ratio. In the fine trench having the high aspect ratio, it is difficult to modify the SOD film or to fill the trench using an SOD process (or spin coating process), thus causing deterioration in the insulation property.
To address such a problem, a technique is known in which a silicon layer is formed on a semiconductor substrate that is exposed on the bottom of the trench corresponding to the device isolation region, using a selective epitaxial growth process, such that the aspect ratio of the trench is mitigated. In this manner, the technique improves modification of the SOD film and enables the trench to be filled by an SOD process, which prevents the insulation property of the device isolation region from being deteriorated, even when the trench is fine and has a high aspect ratio.
However, the aforementioned technique forms the SOD film in the inside of the trench. For this reason, as the high integration of the semiconductor device is further improved and the aspect ratio of the trench becomes higher, this leads to a recurrence of the poor modification of the SOD film and the difficulty of filling the trench by the SOD process. Further, this may cause a problem in that the insulation property of the device isolation region is degraded.